Let \(P_a\)  and \(P_b\)  denote the experimental procedures followed to prepare the states \(\mathinner {|{a}\rangle }\)  and \(\mathinner {|{b}\rangle }\)  to be cloned. As an operational signature of the fact that \(\mathinner {|{a}\rangle }\)  and \(\mathinner {|{b}\rangle }\)  are two pure and, in general, nonorthogonal states, we consider the `test measurements' \(M_a\) , \(M_b\) , with outcomes \(x \in \lbrace 0,1\rbrace \) , giving the operational statistics \(p(M_a|P_a) = p(M_b|P_b) = 1\) . In the quantum formalism, this statistics is reproduced by performing the projective measurements \(\lbrace  \left|a\right\rangle \!\left\langle a\right|, {1}- \left|a\right\rangle \!\left\langle a\right|\rbrace \)  and \( \lbrace  \left|b\right\rangle \!\left\langle b\right|, {1}- \left|b\right\rangle \!\left\langle b\right|\rbrace \)  (with \(x=1\)  corresponding to the first outcome). We will use the notation \(c_{ab} := p(M_{b}|P_{a})\) , which is called `confusability' in Ref. [1]}, for the probability of observing the first outcome of the \(M_b\)  measurement when the system is initialized according to \(P_a\) . Clearly, in the ideal quantum experiment one observes \(c_{ab} = |\!\left\langle a \vphantom{b}| b \vphantom{a}\right\rangle \!|^2\) .
